A solid electrolytic capacitor comprises an anode body, a dielectric body (oxide film) and a cathode and generally has a structure in which an oxide film as a dielectric layer is formed on the surface of a valve-acting metal material as an anode material and a solid electrolyte layer as a cathode is formed thereon.
Conventionally, an aluminum (Al) foil and a tantalum (Ta) sintered element have been widely used as an anode material for a solid electrolytic capacitor. An oxide film is formed on the surface of the anode material by chemical formation, which oxide film functions as a dielectric layer of an electrolytic capacitor.
A method for forming a chemical conversion coating on an Al film is described, for example, in the standards by Electronic Industries Association of Japan “Test method of an electrode foil for an aluminum electrolytic capacitor” (EIAJ/RC-2364A; revised in March, 1999).
A method for forming a chemical conversion coating on a Ta sintered element is described in the standards by Electronic Industries Association of Japan “Test method of a tantalum sintered element for a tantalum electrolytic capacitor” (EIAJ/RC-2361A; revised in February 2000).
As a method for chemical conversion to improve stability of an oxide film, for example, JP-A-H10-223483 (Patent Document 1) describes a chemical conversion method by dipping an aluminum foil in an aqueous solution containing phosphoric acid, boric acid, organic acid or a salt thereof and applying voltage to the foil. JP-A-2000-12396 (Patent Document 2) describes a method for chemical conversion for a Ta sintered body in the presence of an oxidizing agent such as perchloric acids and salts thereof or chromic acid and salts thereof.
Also, JP-A-2000-68159 (WO 99/65043 publication) (Patent Document 3) describes carrying out chemical conversion of a cut surface of an aluminum chemical conversion foil by dipping the foil in an electrolytic solution containing acid such as phosphoric acid, oxalic acid and sulfuric acid and applying voltage to the aluminum bullion as an anode.
Compared to these methods, when niobium (Nb) is used as a valve-acting metal for an anode material, a satisfactory chemical conversion coating cannot be obtained under the same conditions as in the case using Al and Ta.
Nb is a metal having very similar chemical properties to those of Ta and a number of advantages such as a lower specific gravity, larger reserves, higher relative permittivity of an oxide film and lower price compared to Ta. Thus attempts have been made to use niobium as a material for electrolytic capacitors. However, only few researches have been actually made on a niobium electrolytic conversion coating on the assumption that it is similar to a tantalum electrolytic conversion coating since niobium has property values similar to those of Ta, and a satisfactory niobium solid electrolytic capacitor has not yet been available in the market.
Some of the causes for this problem are that the Nb chemical conversion coating is unstable as a dielectric body, has a larger leakage current (LC) than Ta, and exhibits a sensitive irreversible characteristic change against a thermal load. Particularly, in a reflow soldering heat treatment at a temperature of about 200 to 260° C. in a reflow furnace, the alteration of the chemical conversion coating caused even in a short period of the treatment may greatly affect on the capacitor performance.
As one means to solve the problem, JP-A-H11-329902 (U.S. Pat. No. 6,215,652 specification) (Patent Document 4) describes making a chemical conversion coating containing nitrogen by nitriding treatment (300° C.; under nitrogen atmosphere). However, the capacitance of the formation film varies still greatly and a satisfactory performance has not been achieved.
Japanese Patent No. 3965300 (U.S. Pat. No. 6,850,406 specification) (Patent Document 5) focuses on the fact that the cause of the bias dependency of the Nb solid electrolytic capacitor lies in a Nb lower oxide in an amorphous Nb oxide film and describes performing chemical cleaning in addition to nitriding treatment before re-chemical formation but does not describe removal of the treatment liquid after the chemical cleaning or re-chemical formation. When water washing is performed, one can easily expect on commonsense grounds that it will generate additional lower niobium oxide. Accordingly, it is assumed that the method cannot remove the lower niobium oxide completely and the method does not contribute to the decrease of the leakage current, which is an objective of the present invention.